Operating System help please

I am a little confused about an OEM operating system. I just found out I ordered an OEM version of vista and I was just wondering if this will present a problem down the road? I saw a thread a while back but I couldnt find it and the people that posted in it said that if you replace even the harddrive the OS wont boot up without buying a whole new OS. Is this true. I would just like someone to clarify this for me

OEM means that that OS is legally bound to that computer, and you are getting a price break because of that. If you get a new computer, legally, you cannot take the OS with you.

When you start upgrading parts on your computer, Windows Activation may decide that you HAVE a new computer and require reactivation. However, a hard drive upgrade wont do it. Windows Activation uses a point system to calculate what things you CAN change until it decides you have a new computer. Read this for more info.

http://www.aumha.org/win5/a/wpa.php